1. Field of the Invention
The present invention refers to the technical field of communications and particularly to the field of relay-based data transmission.
2. Description of Related Art
Multihop networks have gained recently a lot of interest in the mobile radio research community. They are a promising solution to a major problem that will appear in future mobile radio communication systems (4G=fourth generation), namely the range. Because of the high envisaged center frequency of 5 GHz and the large bandwidth of up to 100 MHz, a significantly increased pathloss and noise power level has to be expected, which translates into a significantly reduced area a base station can cover (as shown in Werner Mohr, Rainer Lüder, and Karl-Heinz Möhrmann, “Data Rate Estimates, Range Calculations and Spectrum Demand for New Elements of Systems Beyond IMT 2000,” in 5th International Symposium on Wireless Personal Multimedia Communications, October 2002, vol. 1, pages 37-46). By introducing relay stations that forward the signal from the base station to far distant mobile stations, this problem can be relaxed. Such relays can be other mobiles or dedicated infrastructure relay stations with fixed power supply. Especially the latter case is of high importance for service providers.
Two main principles of relaying have been identified to be usable in such scenarios: Amplify-and-Forward (AF) and Decode-and-Forward (DF). AF means that a sampled version of the received signal is stored and then retransmitted by the relay station without performing any decoding. This has the big advantage that the relay needs no or only partly knowledge about the structure and coding scheme of the signal. This allows for easy upgrade of a mobile communication network regarding e.g. new coding schemes, without also having to upgrade the relay stations. DF, in contrast to this, means that the signal is decoded at the relay station and re-encoded for retransmission. This has the main advantage that the transmission can be optimized for both links, separately. Furthermore, the signal is regenerated at the relay station, which is not the case for AF. Due to its simple structure, the focus in this application lies on AF.
There exist already a large number of publications on different aspects of multihop networks. A fundamental analysis of cooperative relaying systems was done by
Laneman (J. Nicholas Laneman, David N. C. Tse, and Gregory W. Wornell, “Cooperative Diversity in Wireless Networks: Efficient Protocols and Outage Behavior,” IEEE Transactions on Information Theory, accepted for publication),
who showed that AF schemes provide full diversity gains.
Also Sendonaris (Andrew Sendonaris, Elza Erkip, and Behnaam Aazhang, “User Cooperation Diversity, Part I: System Description,” IEEE Transactions on Communications, vol. 15, pages 1927-1938, November 2003, and
Andrew Sendonaris, Elza Erkip, and Behnaam Aazhang, “User Cooperation Diversity, Part II: Implementation Aspects and Performance Analysis,” IEEE Transactions on Communication, vol. 15, pages 1939-1948, November 2003)
aims on achieving diversity using relay stations. Other issues that were investigated in the past were distributed space-time coding as shown in
Ingmar Hammerstroem, Marc Kuhn, and Boris Rankov, “Space-Time Processing for Cooperative Relay Networks,” in Proc. IEEE Vehicular Technology Conference, October 2004,
capacity enhancements due to usage of relays as virtual antenna arrays as shown in
M. Dohler, J. Dominguez, and H. Aghvami, “Link capacity analysis for virtual antenna arrays,” in Proc. IEEE Vehicular Technology Conference, September 2002, vol. 1, pages 440-443
and rank improvements of a MIMO channel when single-antenna relays are used , as shown in
Armin Wittneben and Boris Rankov, “Impact of Cooperative Relays on the Capacity of Rank-Deficient MIMO Channels” in Proceedings of the 12th IST Summit on Mobile and Wireless Communications, Aveiro, Portugal, June 2003, pages 421-425.
As already mentioned, Amplify-and-Forward (AF) is a simple but effective relaying concept for multihop networks that combines transparency regarding modulation format and coding scheme with ease of implementation. Conventional AF, however, does not take into account the transfer function of the first and the second hop channels. For OFDM based systems, this appears to be sub-optimum. This can especially be concluded from FIG. 6, in which the solid line H1 denotes the transfer function in dependence of the frequency of the first hop (for example a communication channel from a base station to a relay station) and wherein a dashed line H2 denotes the transfer function in dependence of the frequency of the second hop (for example a communication channel from the relay station to a mobile terminal station). As can be furthermore concluded from FIG. 6, subcarriers which have a center or carrier frequency f1 in the frequency bands 600 have a high transmission capacity in the first hop and a poor transmission capacity in the second hop. This means that in these frequency bands strong subcarriers in the first hop would couple into weak subcarriers in the second hop. Thus, in information loss will probably occur in the second hop.
Summarizing, for Amplify-and-Forward (AF) relays, generally the problem appears that the channels for the first and the second hop (for example downlink: 1. Hop is base station-to-relay station channel, 2. Hop is relay station-to-mobile station; uplink vice-versa) are not matched to each other. For OFDM this means that an SNR loss occurs because of the fact that strong subcarrier signals arriving at the relay sometimes couple into weak subcarriers of the second hop channel. Both, the base station and the mobile station, however, are only able to adapt transmission to the overall channel. This means they cannot overcome this disadvantage.
However, relaying for OFDM systems was considered theoretically in
Guoqing Li and Jui Liu, “On the Capacity of the Broadband Relay Networks,” in Proc. 38th Annual Asilomar Conference on Signals, Systems and Computers, CA, USA, November 2004,
but which is unfortunately of no use in the present case having different transfer functions for the first and second hop. No solution to this problem is known up to now.